A 20 nm-thick AlON buffer layer consisting of Al2O3, graded AlON, AlN, and thin Al2O3 amorphous films was used to grow AlN on a sapphire substrate by molecular beam epitaxy with radio frequency plasma for nitrogen source (RF-MBE). Mirror-smooth AlN layers were successfully obtained using this new AlON buffer layer. The total threading dislocation densities of AlN layers are comparable to those reported for the high-quality AlN layers grown by RF-MBE using the conventional low-temperature (LT) buffer layer. (C) 2015 Elsevier B.V. All rights reserved.